The 31-year-old defender has agreed a pre-contract and will sign a permanent deal when his current deal with the Us expires.
He had been offered reduced terms to stay at the Kassam Stadium but opted for a move to Glanford Park.
The penalty specialist was named in the PFA Third Division team of the year this season, and converted five penalty kicks.
He began his career with Leeds United, but went on to play for Doncaster, Darlington, Chester, Brighton and Oxford.